The present invention relates to communications systems, and more particularly, to echo suppression in a bi-directional communications link.
In many communications systems, for example landline and wireless telephone systems, voice signals are often transmitted between two system users via a bidirectional communications link. In such systems, speech of a near-end user is typically detected by a near-end microphone at one end of the communications link and then transmitted over the link to a far-end loudspeaker for reproduction and presentation to a far-end user. Conversely, speech of the far-end user is detected by a far-end microphone and then transmitted via the communications link to a near-end loudspeaker for reproduction and presentation to the near-end user.
At either end of the communications link, loudspeaker output detected by a proximate microphone may be inadvertently transmitted back over the communications link, resulting in what may be unacceptably disruptive feedback, or echo, from a user perspective. This problem is particularly prevalent in the context of hands-free telephony, where the user""s body does not effectively isolate microphone from loudspeaker. Furthermore, impedance mismatches at hybrid junctions in a landline network can result in similarly disruptive echo.
Conventionally, echo suppression has been accomplished using echo canceling circuits which employ adaptive filters to estimate and remove the unwanted echo component of a communications signal. For example, U.S. Pat. No. 5,475,731, entitled xe2x80x9cEcho-Canceling System and Method Using Echo Estimate to Modify Error Signalxe2x80x9d and issued Dec. 12, 1995, describes the use of Least Mean Square (LMS) and Normalized Least Mean Square (NLMS) algorithms in updating Finite Impulse Response (FIR) adaptive filters.
Though such known echo cancellation techniques do remove some echo, audible residual echo will often remain even after cancellation. Such residual echo can, despite its relatively low level, be quite disturbing and should therefore be removed. Today, residual echo suppression is typically accomplished using some form of Non-Linear Processor (NLP) following an echo canceler. See, for example, the above cited U.S. Pat. No. 5,475,731, which describes the use of a conventional center-clipping NLP.
Most known NLPs completely remove those parts of a communications signal containing residual echo. As a result, when both a near-end and a far-end speaker are active (i.e., during double-talk), such known NLPs either pass the residual echo through or remove both the near end speech and the residual echo. Additionally, when known NLPs are used in contexts with complex background noise (e.g., when a car radio is playing while a driver is using a hands-free mobile telephone), the background sounds are also removed along with the residual echo. Though pre-computed comfort noise can be added in an attempt to fill the resulting holes, the processed signal is often choppy and bothersome from a receiving user""s perspective.
Consequently, there is a need for improved methods and apparatus for removing residual echo in communications signals.
The present invention fulfills the above-described and other needs by providing improved frequency domain nonlinear processing techniques. Generally, the filtering characteristic of a nonlinear processor according to the invention is dynamically adjusted by comparing the power spectrum of a residual echo component of a communication signal with that of the overall communication signal itself. More specifically, the filtering characteristic of the nonlinear processor is adjusted so that the nonlinear processor blocks only those signal frequencies in which the residual echo is dominant. Advantageously, a nonlinear processor according to the invention makes full duplex communication possible during the entirety of a conversation, even while actively reducing residual echo.
An exemplary echo suppressor according to the invention includes a processor configured to compute a power spectrum of a communications signal and to estimate a power spectrum of a residual echo component of the communications signal. The exemplary echo suppressor also includes an adaptive filter configured to process the communications signal and to thereby suppress the residual echo component. A filtering characteristic of the adaptive filter is adjusted based on the power spectrum of the communications signal and on the estimated power spectrum of the residual echo.
An exemplary method for suppressing residual echo in a communications signal according to the invention includes the steps of computing a power spectrum of the communications signal and estimating a power spectrum of the residual echo. According to the exemplary method, a filtering characteristic is adjusted based upon the computed power spectrum of the communications signal and upon the estimated power spectrum of the residual echo, and the communications signal is filtered, using the adjusted filtering characteristic, to suppress the residual echo.
According to the embodiments, the filtering characteristic can be adjusted, for example, using spectral subtraction techniques. Alternately, individual coefficients of the filtering characteristic can be set based on direct comparisons of the power in the residual echo with the power in the overall communications signal at the frequencies corresponding to the coefficients.
For stand alone configurations (i.e., where no leading echo canceler is present), estimation of the power spectrum of the residual echo can be accomplished by estimating an attenuation factor of the echo return path and scaling power spectrum samples of the echo source signal by the estimated attenuation factor to thereby provide estimated power spectrum samples of the residual echo. When a leading adaptive echo canceler is present, estimation of the power spectrum of the residual echo can be carried out by computing a power spectrum of an echo component estimate provided by the echo canceler, and scaling the power spectrum of the echo component estimate (based on operation of the adaptive echo canceler) to thereby provide the estimate of the power spectrum of the residual echo.
Scaling of the power spectrum of the echo component estimate can include the steps of determining an echo return loss enhancement of the adaptive echo canceler, and multiplying the power spectrum of the echo component estimate by the echo return loss enhancement to provide the estimate of the power spectrum of the residual echo. Alternatively, scaling of the power spectrum of the echo component estimate can include the steps of determining the echo return loss enhancement of the adaptive echo canceler, computing an average value for the power spectrum of the echo component estimate, multiplying the average value by the echo return loss enhancement to provide a saturation value, multiplying the power spectrum of the echo component estimate by the echo return loss enhancement to provide a scaled spectrum, and clipping the scaled spectrum at the saturation value to thereby provide the estimate of the power spectrum of the residual echo.
Advantageously, the present invention also provides methods and apparatus for adding suitable comfort noise to the filtered (i.e., echo suppressed) communications signal. Generally, a power spectrum of a noise component of the communications signal is estimated, and the comfort noise is created based upon the estimated power spectrum of the noise component and on the prevailing echo-suppression filtering characteristic. According to exemplary embodiments, the comfort noise adds energy at suppressed frequencies (i.e., at frequencies where the filtering characteristic is removing energy and thereby suppressing echo) so that the total energy at those frequencies is equal that of the prevailing noise.